The present invention relates to a process for spherulizing slag and agglomerated fly ash particles, particularly slag and agglomerated fly ash particles resulting from coal-fired generators, an apparatus suitable for carrying out the process, and the spheroidal particles so produced.
Each year many tons of materials such as slag and fly ash resulting from combustion of coal in boilers, hereinafter referred to as coal slag and coal fly ash, found in electric generating plants are produced. In the United States in 1993, for example, over 5.6 million metric tons of coal slag and 43.7 million metric tons of coal fly ash were produced as coal combustion byproducts. The greatest use of such materials is found in roofing granules and as sandblasting materials. Other uses are found in cement and concrete products, snow and ice control, and grouting materials. However, only about 55% of the coal slag and only about 22% of the coal fly ash is incorporated into useful products. The remaining amount is generally disposed of in landfills.
The need to provide additional useful products from such materials and thereby alleviate disposal of these materials in waste storage landfills has long been felt. Forming coal slag or coal fly ash into useful products is considered to be a significant improvement over disposing of such materials in such landfills.
A method for manufacturing rounded vitreous beads is known wherein a feed means which can include a reservoir adapted to hold a fluidized bed of feedstock particles and having at least one overflow under gravity. For manufacture of solid beads, crushed glass cutlet is the recommended feedstock; for cellular beads, a pelletized feedstock containing glass formers and cellulating agent is recommended. The feedstock is delivered to the upper end of a chamber which includes a pair of opposed walls which are spaced apart by a distance less than their breadth, e.g., 15-30 cm, and which are angled to the horizontal so that the feedstock can pass through the chamber under gravity.
In this method, a means for heating the chamber is arranged to heat at least one wall of the pair of opposed walls so that feedstock passing between the pair of opposed walls is heated by radiant heat. The chamber can have segments of increased spacing between the opposed walls from the top of the chamber to the bottom of the chamber as well as differing temperatures in different zones downward through the chamber. For some feedstock compositions, it is desirable to allow the particles to expand while subjected to a temperature in the range of 400xc2x0 C. to 500xc2x0 C., to heat the particles to 800xc2x0 C. to 900xc2x0 C. for spherulization, and to heat them to about 1200xc2x0 C. for partial devitrification.
A process has also been disclosed for producing ceramic powders based on single-or multi-phase metal oxides, including SiO2 compounds, exhibiting a narrow particle size distribution. The raw feed material exhibits a specific surface area of 0.05 up to 500 m2/g and is treated in an indirectly heated drop tube furnace in the form of classified granules exhibiting an average diameter of 10 up to 2500 xcexcm for a period of 0.5 up to 15 seconds at a temperature of 500xc2x0 K. up to 3500xc2x0 K. The raw feed material flows freely into the furnace via a charging device and drops by action of gravity, in a quasi free-falling manner, through the furnace atmosphere, which is oxidizing, inert or reducing, and is cooled. The cooled-off discharged reaction sintered agglomerates are collected and deagglomerated into primary particles by means of an ultrasonic milling device, a sand milt or a jet mill to produce ceramic powder.
Spheroidal particulate material, such as spheroidal cement or smelter slag can be produced by subjecting the particles to a high temperature flame treatment and rapidly cooling them in air. A retention furnace for retaining a molten liquid inorganic material and a nozzle assembly communicated to the retention furnace and capable of scattering therethrough the molten liquid inorganic material are provided. A jet gas entrains the molten inorganic material introduced in the nozzle assembly and scatters the molten inorganic material to cool it. Gas spray means for spraying the jet gas are also provided.
The present invention, in one aspect, provides spheroidal particles of coal slag and coal fly ash. The spheroidal particles have at least a hard outer shell, a diameter preferably in the range of about 0.001 to 5 mm, more preferably in the range of about 0.1 to 1 mm, and comprise SiO2, Al2O3, Fe2O3, FeO, CaO, K2O, and Na2O. Components can also include other oxides such as, for example, oxides of heavy metals, and other materials.
The spheroidal particles of coal slag of this invention have high hardness due to the relatively high amounts of Al2O3. This high hardness allows the spheroidal particles to be used, for example, as shot-peening media. The spheroidal shape of the coal slag particles provides low interparticle friction and a low angle of repose, which enables the particles to be used, for example, as fillers for lubricants or plastic resins, or as flowable construction fill. The spheroidal fly ash particles are resistant to leaching of constituent oxides by water due to the reduction in porosity and surface area that occurs on spherulization. These spheroidal fly ash particles are also resistant to hydrolysis due to the relatively low proportions of alkali metals, i.e., Na2O and K2O, and relatively high proportions of CaO and Al2O3, in the particles. This also provides resistance to leaching of constituent oxides by water.
The present invention, in another aspect, provides a process for spheruling irregularly shaped particles of coal slag or agglomerated coal fly ash, resulting from coal combustion. The process includes the steps of:
(a) providing a drop tube having an upper portion, a central portion and a lower portion;
(b) delivering a feedstock of irregularly shaped particles of coal slag or coal fly ash to the upper portion of the drop tube in a manner such that the particles flow in a substantially vertical downward path through the feed tube as individualized particles;
(c) heating the particles to a sufficient temperature by providing heat to the outer surface of the central portion of the drop tube to cause at least the outer surface of the particles to melt such that a majority, i.e, at least about 50 weight percent, of the particles become spheroidal due to surface tension at the outer surface; and
(d) cooling the particles, preferably in the lower portion of the drop tube, to prevent agglomeration.
The coal slag or agglomerated coal fly ash feedstock, which can range in size from, for example, about 0.001 to 10 mm, preferably from about 0.1 to 1 mm, can be delivered through a feed tube having a small discharge port, having one or more holes, each with a diameter from, for example, at least the maximum particle diameter of the feedstock to at most one-half the diameter of the drop tube, and more preferably, from one to ten times the maximum particle diameter of the feedstock, at the lower end thereof.
The present invention, in a further aspect, provides an apparatus for spherulizing particles comprising
(a) a substantially vertical elongate drop tube;
(b) a feed tube extending into the upper terminal portion of the drop tube and having a substantially closed lower terminal portion with a discharge port therein, the vertical axis of the discharge port being substantially on the vertical axis of the drop tube;
(c) feed means for supplying a feedstock to the feed tube;
(d) vibrating means for intermittently rapping the feed tube to cause discharge of the feedstock from the feed tube in a substantially vertical downward path through the drop tube as individualized particles;
(e) heating means proximate the outer portion of the drop tube and proximate a central portion of the drop tube, the heating means being capable of providing sufficient heat within the drop tube to cause the viscosity of at least the outer portion of the particles to become sufficiently low to allow the surface tension of the particles to spherulize the particles;
(f) cooling means to effect cooling of the spherulized particles such that the particles do not adhere to each other; and
(g) means for collecting the spherulized particles.
The apparatus of the invention is particularly useful with particles having irregular shapes such as, for example, coal slag or agglomerated coal fly ash. The vibrating means can be provided by a magnet and coil, the coil being intermittently powered to cause the magnet to rap the feed tube, or by a solenoid. The feedstock can be provided to the feed tube, for example, by gravity means. The heat can be provided, for example, by a radiant heater, by an electric heating element or gas fired or particle burning heating elements encircling the drop tube, by convection heat such as provided by, for example, direct flame or preheated air, or by induction or dielectric heating of the drop tube and/or the feedstock. A fan can be provided adjacent the lower end of the drop tube, if necessary, to provide suction pressure to overcome any chimney effect caused by heating the central portion of the drop tube.
The present invention, in another aspect, provides a feed system for feeding irregularly shaped particles comprising:
(a) a feed tube oriented substantially vertically and having a substantially closed lower terminal portion with a discharge port therein;
(b) feed means for supplying a feedstock of irregularly shaped particles to the feed tube; and
(c) vibrating means for intermittently rapping the feed tube to cause discharge of the feedstock from the feed tube in a substantially vertical downward path as individualized particles.
The present invention, in another aspect, provides a process for feeding irregularly shaped particles comprising:
(a) feeding irregularly shaped particles to a feed tube oriented substantially vertically and having a substantially closed lower terminal portion with a discharge port substantially centered on the vertical axis of the feed tube;
(b) intermittently rapping the feed tube to cause the particles to discharge from the feed tube in a substantially vertical downward path and at a rate at which the particles remain individualized.